Because your dog is either frightened by the sneeze or he doesn't know what it was. My dog thinks that another dog is barking at him when my brother sneezes! (: hope I helped! Don't forget to follow me!
Maybe, your dog thinks your communicating with him, your sneeze scares him, or he thinks your playing with him. You know how dogs bark when your playing with them? My aunt's dog barks whenever he's being played with :D Btw, hi Susan(: